A cooking unit comprising a glass-ceramic sheet adapted to use as a cooktop and having a coating to provide a decorative effect.
The present invention relates to a decorated glass-ceramic plate as well as to a method of obtaining such a plate, i.e. a method of decorating a glass-ceramic plate. Such plates are especially used as cooktops.
Another object of the present invention is an enamel, useful in particular in the implementation of said decoration method.
The cooking surface defined by a glass-ceramic plate, when it is used as a cooktop, must withstand not only the elevated temperatures employed for cooking without softening or distorting, but also it must withstand large differences in temperature which result from the fact that only one or more parts of the plate are heated. This has led to the use, in this field, of glass-ceramic plates having a coefficient of thermal expansion of zero or near-zero, i.e. generally equal to 0xc2x13xc3x9710xe2x88x927 Kxe2x88x921, and preferably zero.
Glass-ceramics having these characteristics are well known and are widely described in the literature. U.S. Pat. No. 5,070,045 (Comte et al.) describes such glass-ceramics the predominant crystal phase of which is a beta-quartz crystal phase. According to the present invention, decorative materials have been developed which are intended to be used especially, but not exclusively, with such glass-ceramics. It is as such that they can also be used with other low expansion glass-ceramics such as those described in the French patent application No. 97 09912.
Glass-ceramic plates have proved to be extremely satisfactory from the functional point of view as cooktops, but for aesthetic reasons, it was found that it was desirable to decorate the upper surface of said plates.
Enamelling is the decoration method which has proved to be the easiest to implement and which, consequently, is the most generally used. The main points of this technique, which are familiar to the person skilled in the art, have been summarised in the introductory part of the French patent application FR-A-2 701 473.
The initial problem which arises is that of the compatibility of the decorations with the glass-ceramics. In fact, there generally exists a difference in the thermal expansion between the glass-ceramic and the decoration. This difference in expansion of course lowers the mechanical strength of said plate comprising said decoration.
Glass-ceramics have a more than satisfactory inherent mechanical strength, the modulus of rupture (MOR) of the materials actually used being about 180 MPa. However, while the mechanical strength of the actual decorated plates is still adequate, it is significantly reduced and it has revealed to be desirable to obtain higher values, particularly it has revealed to be desirable to obtain decorated glass-ceramic plates having a modulus of rupture (MOR) of at least 120 MPa, preferably of at least about 130 MPa.
Apart from the aesthetic aspect and the problem of the mechanical strength of the decorated plate, a cooktop must also resist staining due to food, it must be easy to clean and must have a smooth surface in order to prevent marks due to contact with metal utensils. These requirements severely limit the potential decoration materials.
For example, a logical candidate was a glass frit that crystallises to produce a low expansion crystal phase, especially a beta-quartz crystal phase. The coatings constituted by these crystallised frits have a very satisfactory modulus of rupture (MOR) but their surface properties are mediocre since the crystals give rise to a surface roughness which is unacceptable, as regards marks left by metallic utensils, and possibilities of easy cleaning.
The U.S. Pat. No. 5,326,728 (Boury et al.), which corresponds to the French patent application FR-A- 2 701 473, describes enamels used in the production of decorative materials. Although these materials have proved to be satisfactory, efforts have been made to produce further improved decorative materials. In particular, these efforts have been directed at obtaining a coefficient of thermal expansion of the decoration which is more compatible with that of the plate this being in order to obtain a greater mechanical strength of the decorated article, and this is indicated by a higher modulus of rupture (MOR).
Thus, an object of the present invention is to provide a glass-ceramic plate which comprises an improved decoration and which has a modulus of rupture (MOR) of greater than 120 MPa whilst at the same time meets the various other requirements of such an article used as a cooktop (requirements recalled above), which has a great flexibility in decoration, which has an improved decoration comprising a mixture of a glass frit with pigments capable of providing a desired decorative effect; as well as to provide a decorative material (an enamel) which can be fired to produce an adherent decoration, especially during the firing of a glass plate to convert the glass into a glass-ceramic; as well as to provide a method of decorating such a glass-ceramic plate.
The present invention does in fact relate to a decorated glass-ceramic plate comprising a glass-ceramic plate having a low coefficient of thermal expansion between xc2x115xc3x9710xe2x88x927 Kxe2x88x921 (20-700xc2x0 C.), preferably near-zero (i.e. equal to 0xc2x13xc3x9710xe2x88x927 Kxe2x88x921 (20-700xc2x0 C.)), and a decoration on the surface of said plate, said decoration comprising a vitrified glass frit containing 10 to 35% by weight of pigments (for 90 to 65% by weight of frit), and said decorated glass-ceramic plate having a modulus of rupture (MOR) of at least 120 MPa.
Preferably, the decorated glass-ceramic plate is constituted by a glass-ceramic the predominant crystal phase of which is a solid solution of beta-quartz and which advantageously has a coefficient of thermal expansion of 0xc2x13xc3x9710xe2x88x927 Kxe2x88x921 (20-700xc2x0 C.).
The present invention also relates to a method of decorating a glass-ceramic plate, especially for generating a decorated cooktop for a cooking appliance, said glass-ceramic plate having a low coefficient of thermal expansion (see above), preferably near-zero (see above), said method comprising:
producing a mixture of 65 to 90% by weight of a glass frit and of 10 to 35% by weight of pigments; said glass frit having a coefficient of thermal expansion of 30-40xc3x9710xe2x88x927 Kxe2x88x921 (0-300xc2x0 C.) (preferably of 30-35xc3x9710xe2x88x927 Kxe2x88x921 (10-300xc2x0 C.)) and a softening point of at least 750xc2x0 C. (preferably of at least 775xc2x0 C.);
applying said glass frit/pigments mixture onto the surface of a glass-ceramic plate or onto the surface of a glass-ceramic precursor glass plate;
firing said plate coated with said glass frit/pigments mixture, to optionally transform said glass plate into a glass-ceramic plate, to vitrify the glass frit in the frit/pigments mixture, to obtain an adherence of the generated decoration with the glass-ceramic; and
cooling said fired plate in order to obtain a decorated glass-ceramic plate having a modulus of rupture of at least 120 MPa.
The present invention is derived from research carried out to produce an improved decorative material for a glass-ceramic plate. In particular, a material was sought which was suitable for glass-ceramic plates the predominant crystal phase of which is a beta-quartz crystal phase and which has a coefficient of thermal expansion of near to zero, preferably of 0xc2x13xc3x9710xe2x88x927 Kxe2x88x921 (20-700xc2x0 C.). The decorative material according to the invention meets this requirement but is not limited to such an application and can be used effectively on other glass-ceramics having a low coefficient of thermal expansion, i.e. situated in the range of 0xc2x115xc3x9710xe2x88x927 Kxe2x88x921 (20-700xc2x0 C.).
The decorative materials used according to the prior art generally comprise pigments dispersed in a glass frit. Said pigments confer to said material its colour and its opacity whereas the glass ensures their mutual binding and their adherence with the glass-ceramic. A glass which is suitable as a frit was described by the Applicant in the U.S. Pat. No. 5,326,728 and consists of a boroaluminosilicate glass which has a coefficient of thermal expansion of about 55xc3x9710xe2x88x927 Kxe2x88x921 (0-300xc2x0 C.) and a softening point of about 675xc2x0 C. (see Composition A of Table II below).
In the course of the research to produce a decorated glass-ceramic having a higher mechanical strength, two factors became apparent which were particularly important for increasing the mechanical strength of a decorated glass-ceramic plate. The first related to the lowering of the coefficient of thermal expansion of the glass frit, and this can be accomplished by means of a frit that can crystallise. However, as indicated above, this approach revealed to be unfruitful due to the fact that the rough surface produced by the crystals is sensitive to marks made by metal utensils and is difficult to clean, and this constitutes an important drawback in the field of the preparation of food. This lack of success led to further studies on glass frits which do not crystallise. The other important factor which was discovered, in a non-obvious way, is that it is important to limit the ionic diffusion which takes place across the interface between the decoration and the glass-ceramic substrate.
The phenomenon of alkali metal ion migration is well known and it was noted that it was not possible to eliminate the presence of the alkali metal ions from the frit without creating other problems despite the presence of said ions being limited. It then appeared important to employ a frit of glass of high viscosity; this being evidenced by a high softening point. In fact, this apparently inhibits the tendency of the alkali metal ions to migrate during heat treatments.
The decorative material according to the present invention essentially consists of 10 to 35% by weight of pigments dispersed in 65 to 90% by weight of glass frit. At least 10% by weight of pigments is necessary to obtain a suitable effect and it is possible to incorporate up to about 35% by weight of pigments. Generally, in order to obtain the decorative effect anticipated, it is not necessary to include more than about 20% by weight of such pigments.
It is possible to use any commercial pigment individually or in combination. The particular pigment or pigments which are employed depend upon the opacity and particular colour desired, or upon another decorative effect desired.
Table I below shows two examples of individual pigments and an example of a combination that can be used. In each case, the pigments are mixed in an amount of 15% by weight with 85% by weight of a glass frit defined below. Table I identifies the pigments by their source, their main constitutive elements and the colour of the coating after firing.
The glass frit employed for the decoration according to the invention advantageously has a coefficient of thermal expansion of 30-40xc3x9710xe2x88x927 Kxe2x88x921 (0-300xc2x0 C.), preferably of at most 35xc3x9710xe2x88x927 Kxe2x88x921, and a softening point of at least 750xc2x0 C., preferably of at least 775xc2x0 C. At the softening point, the viscosity of a glass is 106.5 Paxc2x7s (107.5 poises).
Said included glass is generally a soda potash borosilicate. In fact, it was observed that such glasses possess the characteristics set forth above.
The composition of said included glass, calculated in weight % of oxides, advantageously consists, essentially, of 70-82% SiO2, 12-18% B2O3, 1-3% Al2O3, at most 5% Na2O+K2O and at most 1.2% of at least one fining agent. According to a particularly preferred variant, the composition essentially consists of 76-81% SiO2, 14-15.5% B2O3, 2-2.7% Al2O3, 2.3-3.2% Na2O, 1-1.5% K2O, 0-1% As2O3+Sb2O3 (+meaning  less than  less than  and/or  greater than  greater than ).
Table II below shows a comparison of the composition of a glass frit from the U.S. Pat. No. 5,326,728 cited above (A) with a composition of a preferred glass frit according to the present invention (B). The compositions are indicated in weight % of the glass batch.
A glass frit is prepared by mixing a batch of suitable starting materials and melting it at about 1650xc2x0 C. for about six hours. The molten glass is poured into water wherein it fractures into particles which are dried and ground to a powder having an average particle size of less than about 6 xcexcm.
The pigments are then added which are mixed with the powdered glass frit. The nature of the pigments, and their amounts, depend upon the desired colour and opacity for a particular application. The coating mixture contains at least 10% by weight of pigments, and may contain up to 35% (see above). The balance is constituted by the glass frit. A mixture of 15% pigments and 85% glass frit has proved satisfactory for most of the applications. This mixture should be homogeneous, but no special mixing procedure is required.
The application of a slip or paste by screen printing, which gives a thin uniform coating, constitutes a suitable means of applying said mixture onto a glass-ceramic plate. For such an application, an organic vehicle is added, typically in an amount of 30-50% by weight of the total weight of the slip. The amount of vehicle depends upon the ultimate thickness desired. If a design is desired, the screen printing screen can be prepared in the form of a mask in a known manner.
According to the present invention, it is advantageous that the decorative material can be applied onto the surface of the glass plate which constitutes a precursor for the glass-ceramic plate. After drying, the material on the glass plate can be heat treated according to a ceramming cycle that converts the glass into a glass-ceramic. At the same time, the glass frit, present in the decorating material, softens and bonds to the pigments and onto the surface of the glass-ceramic.
As described in the U.S. Pat. No. 5,070,045, already cited, a suitable ceramming schedule is as follows:
a temperature increase at a rate of 50-80xc2x0 C./min up to the nucleation range, generally located close to the transformation range of the glass,
a temperature increase in the range of nucleation (670-800xc2x0 C.) in about 15-25 min,
a temperature increase up to the crystallisation temperature (900-960xc2x0 C.) in about 15-30 min,
maintaining the crystallisation temperature for 10-25 min, and
rapid cooling to ambient temperature.
Other types of heat treatment or firing may be used in order to ensure the desired effects: ceramming the glass plate+vitrification of the pigment-loaded frit+adhesion of said loaded frit onto the cerammised glass plate. Thus, the decorative material can be applied onto the crude glass plates as described in the French patent application No. 97 09912 and the ceramming cycles, as described in said French patent application No. 97 09912, can be implemented in order to obtain decorated glass-ceramic plates of the invention based on the glass-ceramic plates according to the French patent application No. 97 09912. The maximum temperatures attained during said ceramming cycles may attain values as high as 1070xc2x0 C. In said French patent application No. 97 09912, ceramming cycles are also described which comprise several successive cycles. In such cases, it is preferred to apply the glass frit/pigments mixture (precursor of the desired decoration) before the last of said successive cycles such that said mixture undergoes one sole heat treatment only.
Hence, it is understood that the expression  less than  less than  glass-ceramic precursor glass plate  greater than  greater than , as used in the present description and the annexed claims, incorporates both the crude glass plate as well as the glass plate undergoing ceramming.
For some applications, it may be desirable, or even necessary, to apply the material onto the already cerammised glass-ceramic plate. In this case, the coating can be applied in the same manner as in the case of a precursor glass plate, but directly onto the glass-ceramic plate. This double firing procedure (1xc2x0 ceramming; 2xc2x0 treatment of the glass frit/pigments mixture) may be desirable for adjusting each firing cycle. In this case, the material situated on the glass-ceramic may be fired for about 15 minutes at a temperature in the range of 920-960xc2x0 C.
Another object of the present invention is the enamel used for the implementation of the decorating method described above, used for obtaining the decorated glass-ceramic plates of the invention. Said enamel is novel per se, and constitutes another object of the present invention. Characteristically, the enamel comprises 10 to 35% by weight of pigments in 65 to 90% by weight of a specific glass frit. Said glass frit has a coefficient of thermal expansion of 30-40xc3x9710xe2x88x927 Kxe2x88x921 (0-300xc2x0 C.), preferably of 30-35xc3x9710xe2x88x927 Kxe2x88x921 (0-300xc2x0 C.) and a softening point of at least 750xc2x0 C., preferably of at least 775xc2x0 C. Advantageously, the composition of said glass frit, calculated in weight % of oxides, essentially consists of 70-82% SiO2, 12-18% B2O3, 1-3% Al2O3, at most 5% Na2O+K2O and at most 1.2% of at least one fining agent. Particularly preferably, said composition essentially consists of 76-81% SiO2, 14-15.5% B2O3, 2-2.7% Al2O3, 2.3-3.2% Na2O , 1-1.5% K2O and 0-1% As2O3+Sb2O3.